The MPEG-2 standard represents video sequences by video elementary streams. Video elementary streams represent the raw output of a video encoder that compresses the video sequences and can be associated with other elementary streams, such as an audio elementary stream. The video elementary stream and associated other elementary streams after PES packetisation form what is known as a program. The PES streams so formed from Video and associated other streams are packetized into fixed length transport packets for transmission over a communication medium. The transport packets are transported as a stream (a transport stream) over a communication channel for decoding by a decoder.
The transport stream can include any number of programs multiplexed together for transport over a communication channel. Each program may use a different compression factor and bit rate that can change dynamically, even though the overall bit rate stays constant. The foregoing is known as statistical multiplexing. A decoder must be able to change from one program to the next and correctly select the appropriate video, audio, and other associated elementary streams.
The decoder demultiplexes the transport packets using a combination of parameters and tables. The transport packets include a header with packet identification parameter. The packet identification parameters correspond to entries in a program access table and Program Map Table that is present in the transport stream.
Once the video and audio elementary streams are demultiplexed and extracted from the transport stream, the video and audio elementary streams can be stored separately for future playback. The video and audio elementary streams are usually stored in a high capacity memory, such as a hard disc drive.
Some trick mode schemes now allow for simultaneous playback of multiple video elementary streams. For example, a feature known as Picture in Picture includes a primary video with another video displayed in a small section of the screen. The foregoing can be achieved in decoders with multiple playback channels. Additional playback channels are needed because after the video elementary streams are extracted from the transport stream, the video elementary streams are not readily distinguishable from one another.
However, addition of playback channels to decoder systems requires additional hardware and increases costs.
Further limitations and disadvantages of convention and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings.